This invention relates to an optical information recording-reproducing apparatus of the two-light-source type which is provided with a light source for recording and a light source for reproduction separately.
Various information mediums such as a disk-like medium, a card-like medium and a tape-like medium are known as optical information mediums for effecting the recording and/or reproduction of information thereon by the use of light. These optical information recording mediums include mediums capable of recording and reproduction, and mediums capable of reproduction only. To record information on a medium capable of recording, information tracks are scanned by a light beam modulated in accordance with recording information and stopped down into a minute spot, and the information is recorded as an optically detectable information pit row.
Also, to reproduce information from a recording medium, an information pit row on an information track is scanned by a light beam spot of such a degree of predetermined power that recording is not effected on the medium, and light reflected from or transmitted through the medium is detected.
An optical head used to record and/or reproduce information on thee recording medium is made movable relative to the recording medium in the direction of the information tracks thereof and a direction transverse to said direction, and by this movement, the information track scanning of the light beam spot is effected. As a lens for stopping down the light beam spot in the optical head, use is made, for example, of an objective lens. This objective lens is held so as to be movable independently with respect to the optical head body in the direction of the optical axis thereof (the focusing direction) and a direction orthogonal to both of the direction of the optical axis and the direction of the information tracks of the recording medium (the tracking direction). The holding of such an objective lens is accomplished generally through an elastic member, and the movements of the objective lens in said two directions are generally driven by an actuator which utilizes a magnetic interaction.
Now, among the above-mentioned optical information recording mediums, the card-like optical information recording medium (hereinafter referred to as the optical card) has a great expected demand as an information recording medium of relatively large capacity which is compact and light in weight as well as convenient to carry.
FIG. 1 of the accompanying drawings shows a schematic plan view of a postscript type optical card, and FIG. 2 of the accompanying drawings shows a fragmentary enlarged view thereof.
In FIG. 1, a number of information tracks 2 are arranged on the information recording surface of the optical card 1 in parallelism to one another in the direction of arrows L and F. Also, a home position 3, which provides the reference position of the access to the information tracks 2, is provided on the information recording surface of the optical card 1. The information tracks 2 are arranged as indicated by 2-1, 2-2, 2-3, . . . in order from the home position 3, and as shown in FIG. 2, tracking tracks are successively provided adjacent to these respective information tracks as indicated by 4-1, 4-2, 4-3, . . . . These tracking tracks 4 are used as a guide for auto tracking (hereinafter referred to as AT) which controls a light beam spot so as not to deviate from a predetermined information track when the light beam spot scans during the recording or reproduction of information.
This AT servo is accomplished by detecting the deviation (AT error) of the light beam spot from the information track in the optical head, negatively feeding back the detection signal to the tracking actuator, moving the objective lens relative to the optical head body in the tracking direction (the direction of arrow D, and causing the light beam spot to follow a desired information track.
When the information tracks are scanned by a light beam spot during the recording or reproduction of information, auto focusing (hereinafter referred to as AF) servo is effected to make the light beam into a spot of suitable size (focus) on the surface of the optical card. This AF servo is accomplished by detecting the deviation (AF error) of the light beam spot from its in-focus state in the optical head, negatively feeding back the detection signal to the focusing actuator, moving the objective lens relative to the optical head body in the focusing direction and focusing the light beam spot on the surface of the optical card.
In FIG. 2, S1, S2 and 3 designate light beam spots, and the light spots S1 and S3 are used to effect tracking. The light spot S2 is used to effect focusing, the preparation of information pits during recording, and the reading-out of the information pits during reproduction. Also, in the respective information tracks, 6-1, 6-2 and 7-1, 7-2 denote left address portions and right address portions, respectively, subjected to preformat, and by reading out these address portions, the identification of the tracks is accomplished. The reference numeral 5 (in FIG. 2, 5-1 and 5-2 correspond thereto) designates data portions in which predetermined information is recorded.
Here, the optical information recording system will be described briefly. The optical information recording system broadly includes two types. One type is the single light source type in which recording and reproduction are effected by the use of one and the same light source, and the other type is the two-light-source type in which recording and reproduction are effected by the use of two different light sources. The two-light-source type, as compared with the single light source type, is said to be advantageous in terms of the deterioration of reproducing light, high speed, etc.
FIG. 3 of the accompanying drawings shows a schematic view of an optical head optical system of the two-light-source type. The two-light-source type adopts discrete light sources for recording and reproduction, thereby making the prevention of the deterioration of reproducing light and high-speed recording possible.
In FIG. 3, the reference numerals 21 and 22 designate semiconductor lasers which are light sources. The semiconductor laser 21 emits lights of wavelength 780 nm, and the semiconductor laser 22 emits light of wavelength 830 nm. The reference numerals 23 and 24 denote collimator lenses, the reference numeral 25 designates a diffraction grating for dividing a light beam, the reference numeral 26 denotes a dichroic prism designed to transmit light of 780 nm of P-polarized component therethrough and reflect light of 830 nm, the reference numeral 27 designates a beam shaping prism, and the reference numeral 28 denotes a polarizing beam splitter. The reference numeral 29 designates a quarter wavelength plate, the reference numeral 30 denotes an objective lens, the reference numeral 31 designates a band-pass filter transmitting only light of 780 nm therethrough, the reference numeral 32 denotes a stopper, the reference numeral 33 designates a topic lens, and the reference numeral 34 denotes a photodetector.
Light beams emitted from the semiconductor lasers 21 and 22 become divergent light beams and enter the collimator lenses 23 and 24, respectively, and are modified into parallel light beams by these lenses light of 780 nm further enters the diffraction grating 25, and is divided into three effective light beams (0-order diffracted light and .+-.1st-order diffracted lights) by this diffraction grating. The light beam of 780 nm and the light beam of 830 nm enter, as P-polarized components, dielectric multi-layer film laminated on the adhesively secured surface of the dichroic prism 26 having a spectral characteristic as shown in FIG. 4 of the accompanying drawings. The dichroic prism 26, as is apparent from FIG. 4, has the characteristic of transmitting light of 780 nm of P-polarized light therethrough and reflecting light of 830 nm. Therefore, the light beam of 780 nm is transmitted and the light beam of 830 nm is reflected, and the two light beams emerge from the dichroic prism 26 as they are combined together. The light beam passed through this dichroic prism 26 is shaped into a predetermined light intensity distribution by the beam shaping prism 27, and then enters the polarizing beam splitter 28. The polarizing beam splitter 28, as shown in FIG. 5 of the accompanying drawings, has the spectral characteristic of transmitting P-polarized light therethrough and reflecting S-polarized light, and transmits the light beams of two wavelengths therethrough because these light beams are P-polarized components. Then, these light beams of two wavelengths are converted into circularly polarized light when they are transmitted through the quarter wavelength plate 29, and converged by the objective lens 30. The light beam of 780 nm is applied onto the optical card 1 as three minute beam spots S1 (+1st-order diffracted light), S2 (0-order diffracted light) and S3 (-1st-order diffracted light), and these beam spots are used as reproducing light and signal lights for AT and AF control. Also, the light beam of 830 nm is applied onto the optical card 1 as a minute beam spot of S2 (0-order diffracted light) and used as recording light.
The positions of the light beam spots on the optical card 1 are similar to those shown in FIG. 2, and the light beam spots S1 and S3 lie on the adjacent tracking tracks 4 and the light beam spot S2 lies on the information track 2 between said tracking tracks. As regards the positional relation between the light beam spot S2 of 780 nm and the light beam spot S2 of 830 nm, it is better for the light beam spot S2 of 830 nm, which is the recording light, to lie somewhat in the direction of travel, but the positional relation is free in principle and here, these two light beam spots coincide in position with each other. Thus, the reflected lights from the light beam spots formed on the optical card 1 pass through the objective lens 30 and are thereby made substantially parallel to one another and are again transmitted through the quarter wavelength plate 29, whereby they become light beams having their direction of polarization rotated by 90.degree. with respect to that when they have entered. Therefore, they enter the polarizing beam splitter 28 as S-polarized beams, and since this splitter reflects S-polarized light as previously described, the light beams are reflected toward the band-pass filter 31. Only the light in the vicinity of 780 nm is transmitted through, and the lights of the other wavelengths are reflected by the band-pass filter 31 having a spectral characteristic of transmitting only the light in the vicinity of 780 nm therethrough as shown in FIG. 6 of the accompanying drawings, whereby only the light of 780 nm is directed as a light for a signal to the detecting optical system. The light transmitted through the band-pass filter 31 is converged by the toric lens 33 and enters the photodetector 34. The photodetector 34 is of a construction as shown in FIG. 7 of the accompanying drawings, and effects tracking control by signals received by light receiving elements 11 and 13, and effects focus control and reproducing signal detection by a signal received by a light receiving element 12 which is a four-division element.
However, in the example of the optical head shown in FIG. 3, two semiconductor lasers are required, and this leads to the problem that the rate the optical system occupies becomes great. That is, compactness and light weight are required of the optical head, whereby the higher speed of access time is desired. However, as previously described, an increase in the number of optical parts has hampered the compactness of the optical head and has hindered the higher speed.
The present invention has been made in view of such circumstances and the object thereof is to provide an optical information recording-reproducing apparatus in which in spite of two light sources being used, the number of parts can be effectively decreased, whereby an optical head can be made compact and light in weight.
To achieve the above object, there is provided an optical information recording-reproducing apparatus which has two light sources emitting a light beam for recording and a light beam for reproduction differing in wavelength from each other and records and/or reproduces information on a recording medium by the use of the light beams from said light sources, characterized in that a beam shaping prism for shaping the light beams from said two light sources into predetermined intensity distributions is comprised of a division prism comprising at least three prisms cemented together, a polarization dividing surface transmitting therethrough the light beams from said light sources and reflecting the reflected light from said recording medium is formed on at least one of the dividing surfaces of said division prism, and a wavelength dividing surface, for directing the reproducing light beam of the reflected light from said recording medium to a detecting optical system, is formed on at least one other dividing surface.